1. Field of the Invention
The present invention relates to a photodetector arrangement applicable to a light beam scanning apparatus and, particularly, to a light beam scanner for sweeping a light beam to read out or record image information.
2. Description of the Prior Art
In a prior art light beam scanning apparatus, a light beam emitted from a light source such as a laser is reflected and swept by a mirror which is swung by a galvanometer. Where such a scanner is applied to an image information recording system, an acoustooptic modulator (AOM) may be used in order to modulate the light beam with image information signals so that the modulated beam scans a photosensitive recording medium to record the image information thereon. Where it is applied to an image information read-out or reproducing system, on the other hand, a medium carrying image information thereon may be scanned with the light beam to allow the information to be read thereoutof by sensing light transmitted therethrough or reflected therefrom.
Particularly, when a radiation image stored in a stimulable phosphor sheet is read out, the laser beam is used as stimulating rays, and the read out of the radiation image is performed by scanning the sheet with the laser beam and photoelectrically detecting the light emitted from the sheet to obtain signals associated with the radiation image. Whether the light beam scanner of the type as described above is applied to an image information recording system or to an image information read out system, it is desirable that the light beam is swept at a predetermined speed. In an image information recording system, for example, an image with a uniform quality can be recorded if the exposure time per pixel is constant. In an image information read out system, on the other hand, an image can uniformly be produced if integration time per pixel is constant.
It will be seen from the above that maintaining the scan speed of the light beam constant is an important consideration in achieving high quality images. However, it is difficult to maintain the scan speed strictly constant due to the inherent structural conditions of the apparatus. Usually, a signal indicative of a position of the light beam is generated in response to a movement of a galvanometer and an image signal is recorded or integrated in synchronism with the position signal. The generate the position signal, a photodetector is employed and the photodetector has to provide a stable signal entailing a minimum of noise or fluctuation. A prior art photodetector to generate the position signal uses a light conductor in the form of an elongate, cylindrical member made of acrylic resin or the like. On the side face of the light conductor, there is provided optical grids or gratings along the longitudinal axis thereof. The optical grids or gratings are formed by vapor-depositing, for example, chromium in the form of stripes. A photosensitive device such as a photomultiplier is located at either end of the light conductor. A light beam sweeping arrangement is so arranged as to cause a light beam to scan the array of optical grids. The light entering the light conductor through the grid array is partly allowed to propagate over the light conductor to reach the photosensitive device, while undergoing repeated reflection and diffusion thereinside. In such a prior art photodetector, as the scanning beam is incident on a part of the light conductor which is adjacent to the photosensitive device, the amount of incident light on the device may sometimes increase to a significant level. Then, the reference signal developed from the photosensitive device may increase to an extremely high level. The signal level progressively decreases in inverse proportion to a distance between the photosensitive device and the scanning beam. Generally, the ratio between maximum and minimum values of the signals which are desirable as reference signals is not larger than several times. Applying a signal having a larger level difference than the above-mentioned to a wave shaping circuit of the apparatus as a reference signal is undesirable because it would make the design of the wave shaper difficult and the stability of operation poor.